It looks like you added the -j15 to the 10+j4. The problem with doing that is that 10+j4 is for a resistance of 10 in with a reactance of +j4. Your solution would be correct if you were to add the capacitor in series with the other two.

I should do the combo in parallel like combining resistor in parallel but I am not sure how to do this since the capacitance does not have a magnitude. For example, would I just use
0+j4 in calculations?

Let me know how they tell you to work the problem. I derived these formulas myself. I don't know the official method. When I was doing bench level RF design I used to work these in my head for practice.